The oxidation of alkyl aromatic compounds, e.g., toluene and xylenes are important commercial processes. A variety of oxidation products may be obtained including aromatic carboxylic acids such as terephthalic acid (1,4-benzenedicarboxylic acid) which is used, for example, in the polymer industry.
U.S. Pat. No. 2,833,816 discloses processes for oxidizing aromatic compounds to the corresponding aromatic carboxylic acids. A process for the liquid phase oxidation of alkyl aromatic compounds uses molecular oxygen, a metal or metal ions, and bromine or bromide ions in the presence of an acid. The metals may include cobalt and/or manganese. Exemplary acids are lower aliphatic mono carboxylic acids containing 1 to 8 carbon atoms, especially acetic acid.
U.S. Pat. No. 6,355,835 discloses a process for the preparation of benzene dicarboxylic acids by liquid phase oxidation of xylene isomers using oxygen or air by oxidising in the presence of acetic acid as solvent, cobalt salt as catalyst and an initiator. The oxidation step is followed by flashing the reaction mixture to remove volatile substances and cooling and filtering to get crude benzene di-carboxylic acid as a solid product and filtrate. Recrystallizing the crude benzene di-carboxylic acid to obtain at least 99% purity and recycling of the filtrate are also disclosed.
U.S. Pat. No. 7,094,925 discloses a process for the oxidation of an alkyl-aromatic compound, wherein the aromatic compound is admixed with an oxidising agent or sulfur compound in the presence of an ionic liquid. Air, dioxygen, peroxide, superoxide, any other form of active oxygen, nitrite, nitrate, nitric acid or other oxides (or oxyhalides) of nitrogen (hydrate or anhydrous) are preferably used as the oxidising agent. The process is usually under Bronsted acidic conditions. The product of the oxidation reaction is preferably a carboxylic acid or ketone or an intermediate compound in the oxidation such as an aldehyde, or alcohol. The oxidation is preferably performed in an ionic liquid containing an acid promoter such as methanesulfonic acid.
US 2009/0326265 A1 discloses a process for preparing an aromatic polycarboxylic acid by liquid phase oxidation of a di- or tri-substituted benzene or naphtalene compound, the process comprising a step of contacting the aromatic compound with an oxidant in the presence of a carboxylic acid solvent, a metal catalyst and a promoter in a reaction zone, wherein the promoter is an ionic liquid comprising an organic cation and a bromide or iodide anion. Advantages of this process include high conversion without severe corrosion problems otherwise associated with halogen-containing compounds as promoter. The process does not necessitate the use of special corrosion-resistant material or liners in the process equipment; thus offering savings on investment and maintenance costs and increasing plant reliability. The process of the invention is especially suited for production of terephthalic acid from p-xylene.
It is also known in the art that the oxidation products such as aromatic aldehydes, aromatic alcohols, aromatic ketones, and aromatic carboxylic acids may solidify or crystallize at oxidation conditions. Thus, mixtures of oxidation products are produced which may require further processing to increase the purity of the desired product. For example, in the production of terephthalic acid, the solid oxidation product is often referred to as crude terephthalic acid as it contains impurities including color bodies, para-toluic acid, and 4-carboxybenzaldehyde (4-CBA) in amounts which make it unacceptable in many polymer applications.
The para-toluic acid and, 4-CBA content of the solid oxidation product may be reduced by washing or dissolving the solid oxidation product in water or other solvents such as acetic acid and methanol and re-solidifying or re-crystallizing the terephthalic acid. However, such processes are typically unable reduce the 4-CBA content sufficiently to meet polymer grade or purified terephthalic acid requirements. To meet purified terephthalic acid quality requirements, conventional purification processes typically reduce the 4-CBA content by reacting a solution of dissolved crude terephthalic acid with hydrogen at hydrogenation conditions usually including a catalyst comprising palladium and carbon. This converts the 4-CBA to para-toluic acid which is then removed by washing and/or dissolution and re-solidifying processes as described above.
U.S. Pat. No. 7,692,036 discloses an optimized process and apparatus for more efficiently and economically carrying out the liquid-phase oxidation of an oxidizable compound. Such liquid-phase oxidation is carried out in a bubble column reactor that provides for a highly efficient reaction at relatively low temperatures. When the oxidized compound is para-xylene and the product from the oxidation reaction is crude terephthalic acid (CTA), such CTA product can be purified and separated by more economical techniques than could be employed if the CTA were formed by a conventional high-temperature oxidation process.
There remains a need in the art for alternate processes that reduce the contaminant levels of crude terephthalic acid. Processes that produce purified terephthalic acid without requiring a hydrogenation and subsequent are desirable as they provide significant capital and operating cost savings.